Film cameras have been developed with electromechanical systems that support automatic functions such as film winding, film rewinding, exposure control, electronic flash, etc., all controlled by a controller such as a microprocessor. Electrical energy is provided to such electromechanical systems and the controller by a power supply. Most often, the power supply is a chemical battery of conventional design that stores a fixed amount of potential energy and releases this potential energy in the form of electricity. As this electricity is used, the amount of power remaining in the power supply is reduced. After extended operation, the potential energy stored in the power supply can be reduced to a level that is insufficient to allow the camera to reliably perform certain camera operations.
The amount of potential energy stored in the power supply can be determined based upon the difference of potential or voltage between electrically positive and negative terminals of the power supply. As potential energy in the power supply is reduced, the difference in potential at the terminals lowers. Accordingly, cameras are known that monitor voltage levels between the terminals of the power supply and provide a warning when voltage levels at the terminals reach a predetermined low level.
However, it can also be useful to prevent the camera from attempting to perform functions that cannot be reliably performed when the camera is in operation. The cameras of the prior art employ various general strategies to prevent camera mis-operation caused by low levels of available energy in a battery.
One strategy is to modify the operation of certain camera elements in order to ensure that the operation of the camera components does not consume so much power as to interfere with the operation the camera. For example, U.S. Pat. No. 5,023,470, filed by Onozuka et al. on Apr.18, 1989 shows an electronic flash charging circuit for use with a camera having a power source common to a microcomputer that controls a plurality of camera functions and to an electronic flash. The charging circuit has a booster circuit for boosting a charging voltage with which a main capacitor of the electronic flash is charged and a controller that causes the booster circuit to operate intermittenly. This intermittenly charges the main capacitor so that charging the capacitor does not lower the battery voltage below the level necessary to support operation of the microcomputer.
Another strategy involves using the voltage level at the battery to determine whether the operation of the camera microprocessor will be altered by the performance of particular camera functions. U.S. Pat. No. 5,027,150, entitled “Camera” filed on Jun. 25,1991, by Inoue et al. describes a camera system that detects a battery voltage that is below a threshold and suspends camera operation in response thereto. The camera described in the '150 patent also stores data that is in the microprocessor in a backup memory so that such data is not lost when the camera batteries are changed. In still another example of this type, U.S. Pat. No. 4,126,874 entitled “Power Supply Circuit for A Camera”, filed by Suzuki et al. on Dec. 20, 1976 describes a power supply circuit that uses a delayed testing scheme to test battery voltage levels. In this patent, camera operation is disabled where the voltage levels detected after the delay are below a threshold. This delayed testing is used where the battery response to the testing is such that the battery responds more accurately to testing after the battery has been used for a period of time.
Yet another strategy involves testing the battery under load to determine whether the battery has sufficient energy to support a maximum load that may occur during camera operation. Where the monitoring indicates that the load is below the maximum, functions associated with the maximum load are disabled. For example, Suzuki et al. U.S. Pat. No. 4,502,744 describes a battery check procedure that applies an actual load on the camera battery that simulates the maximum load that can be placed on the battery by one of the camera components. The voltage at the power supply is monitored during this maximum load. If this voltage is below a threshold, photography is inhibited.
A further strategy involves determining whether particular functions can be performed and disabling those functions when the camera battery does not have enough energy to perform those functions. For example, U.S. Pat. No. 5,500,710 entitled “Source Voltage Monitor for A Photographic Camera”, filed by Saito et al. on Dec. 15, 1994 describes a system that applies a load to a battery and tests the battery voltage levels under load prior to release of the shutter to determine whether there is sufficient power in the camera to effect shutter release. Shutter release is prohibited where the voltage levels indicate that there is insufficient power in the camera battery to properly release the shutter. Similarly, U.S. Pat. No. 4,611,989 entitled “Voltage Detecting Device” filed by Matsuyama on Feb. 13, 1985 describes a voltage detector that measures voltage during movement of a leader screen on a camera shutter so that an accurate determination can be made as to whether there is sufficient energy in a power supply to effect a normal release of a follower screen. In these patents, shutter release is prohibited where the voltage levels indicate that there is insufficient power in the camera battery.
The systems described above show various means for insuring the particular camera operations do not create a risk of unusual operation by testing the battery to determine whether there is sufficient power to perform one or another of the camera operations. In most automatic cameras, the photographic process involves many operations each of which consumes power. Thus, while there may be sufficient power in the power supply to provide reliable performance of one camera operation, there may not be sufficient power in the power supply to provide reliable performance of that camera operation after other precursor operations have been performed as the camera operations are executed to capture an image. Thus, testing a camera power supply to detect whether there is sufficient energy in the camera power supply to perform a particular camera function does not always provide an accurate indication as to whether there is sufficient energy to perform the entire set of camera operations.
The alternative strategy of testing voltage levels at a power supply during the photographic process and selectively disabling certain camera functions as is described in certain of the above-cited patents, can be problematic. This is because many photographers can be confused when a camera ceases operation during a portion of a photographic process and can draw the wrong conclusion that the camera mechanical systems have failed when the source of the problem is exhaustion of the power supply.
Thus, what is needed is a camera and method for controlling a camera having a new control strategy that addresses these considerations.